Photovoltaic devices such as solar cells are capable of converting solar radiation energy into usable electrical energy. Energy conversion occurs as the result of what is well known in the solar cell field as the "photovoltaic effect". Two basic steps are involved in the photovoltaic effect. Initially, solar radiation absorbed by the semiconductor generates electrons and holes. Secondly, the electrons and holes are separated by a built-in electric field in the semiconductor solar cell device. This separation of electrons and holes results in the generation of an electrical current. A built-in electrical field can be generated in a solar cell by, for example, a P-N junction. The electrons generated at the P-N junction flow towards the N-type material to which an ohmic contact must be formed to collect the electrons. The ohmic contact may be either a metallic grid system or a transparent highly conducting N-type semiconductor.
The standard grid ohmic contact consists of a six fingered geometric pattern of a metal such as gold, chromium, silver, etc. U.S. Pat. No. RE. 28,610 teaches an improved finger geometric pattern for collecting photocurrent. However, a grid system suffers from the shadowing effect of the metal fingers, which reduces the total area of the solar cell which can be exposed to solar radiation, and thereby reduces the total collectable photocurrent.
Transparent metals or metal oxides, such as indium tin oxide, tin oxide, or copper oxide, do not shadow the solar cells due to their transparency; however, the metal oxides have higher resistivities than the metal fingers. This limits the usable voltage available from the photocell. In addition, the bandgap of the oxides may be sufficiently low to shield or filter out some of the solar radiation. Furthermore, tin oxide and indium tin oxide tend to chip or flake when applied in a sufficient thickness to reduce the resistance of the coating.
Thus, it would be desirable to have a transparent metal or semiconductor coating with a low resistivity, which is transparent to visible and infrared solar radiation, and has a high thermal conductivity and stability to elevated temperatures for use in solar cells with solar concentrators.